Heat exchange is a unit procedure essential for distillation process systems for organic compounds represented by petrochemical plants and for reaction process systems for organic compounds represented by olefin polymerization reaction. In a heat exchanger for performing heat exchange, however, fouling occurs sometimes to cause obstacles such as a reduction in the heat exchange efficiency of the heat exchanger, an increase in the pressure of a pump, an increase in the energy consumption of the pump, a reduction in the efficiency of separation of components and a reduction in the flow rate of cooling water. As a result, the distillation operation and manufacturing operation are made unstable, and the operation should be terminated in the worst case.
For preventing the fouling of heat exchangers in petroleum refining facilities as a typical example of petrochemical plant, JP-A No. 55-129490 discloses a method which comprises adding 5 to 99 ppm polyalkylene amine to a hydrocarbon stream. However, the polyalkylene amine may be toxic to the human body, and thus its influence on the health of an operator and its contamination of products are worried about. JP-A No. 2004-43615 discloses a method of removing fouling materials by adding a dialkyl sulfide to raw oil, but its influence on the health of an operator and its contamination of products are worried about as well.
Now, attention is paid to the reaction process systems for organic compounds. Polyolefins such as polyethylene, polypropylene, ethylene-α-olefin copolymers and propylene-α-olefin copolymers are produced by a wide variety of known process such as a liquid phase polymerization process and gas-phase polymerization process. Among these polymerization processes, the gas-phase polymerization process gives (co)polymers in the form of particles, and unlike the liquid phase polymerization process, does not need steps such as precipitation and separation of polymer particles from a polymer solution, thus simplifying the production process, and therefore, production of polyolefins by the gas-phase polymerization is extensively carried out in recent years.
In the liquid phase polymerization process and gas-phase polymerization process, polyolefins are produced by (co)polymerizing olefins in the presence of a solid catalyst such as a solid titanium-based Ziegler-Natta catalyst disclosed in, for example, JP-A No. 58-83006 and JP-A No. 7-25946 and a carrier-supported metallocene catalyst disclosed in JP-A No. 2000-297114. In those processes, however, the amount of the product is increased, polymerization heat is usually increased. For removing the heat of polymerization, a method of removing heat in liquid phase polymerization by withdrawing the polymerizing solution once through a pipe etc. to the outside of the system, passing the solution through a heat exchanger (also referred to hereinafter as “intercooler”) to cool it, and returning it to the polymerization system may be adopted. There is also employed a method wherein a part of a gas composed of hydrocarbons such as unreacted monomers (also referred to as “hydrocarbon-containing gas”) is withdrawn continuously from a gaseous phase during liquid phase polymerization or from the top of a reactor during gas-phase polymerization, then the hydrocarbon-containing gas is cooled in a heat exchanger to remove heat of polymerization, and the gas (and a partially liquefied gas) is returned as polymerizable monomers to the polymerization reactor. However, fouling in the heat exchanger for removing polymerization heat is a serious problem, but at present there is no efficient method of preventing fouling.
Particularly in the gas-phase polymerization process, monomer(s) is (are) fed via a distribution plate from the bottom of a polymerization reactor, whereby a solid catalyst and solid particles consisting of formed polyolefin are fluidized to form a fluidized bed wherein polymerization reaction proceeds. Produced polyolefin is discharged continuously or intermittently from the reactor. However, solid catalyst particles and polyolefin particles differ from one another in particle diameter and properties and are hardly fluidized uniformly under a certain environment in the gas-phase polymerization process. As a result, the hydrocarbon-containing gas withdrawn from the top contains fines of the solid catalyst and polyolefin particles in larger amounts than by the liquid phase polymerization process. The solid catalyst and polyolefin particles may adhere to the heat exchanger to permit undesirable polymerization reaction and side reaction to proceed in the heat exchanger, thus causing a significant reduction in the ability of the heat exchanger to remove heat. As a result, the production speed is reduced, and thus there is a problem that production should be periodically suspended to clean the heat exchanger and remove clogging.
In the gas-phase polymerization process, a solid catalyst such as a solid titanium Ziegler-Natta catalyst and a carrier-supported metallocene catalyst is used, and there is also another problem that the solid catalyst is often poor in fluidity and is charged very easily, thus aggregating the catalyst or adhering to a polymerization reactor wall, to generate a lump of the polymer in the polymerization reactor. By carrying a surfactant on the solid catalyst, the problem of adhesion to the polymerization reactor wall can be solved to a certain extent (for example, JP-A No. 2000-313717 and JP-A No. 2000-313716), but the fouling of a heat exchanger cannot be solved. The polyalkylene amine and dialkyl sulfide used for preventing the fouling of a heat exchanger in a petroleum refining plant is poisonous to the olefin catalyst, and thus a reduction in the reaction efficiency of olefin polymerization reaction and contamination of products therewith are worried about.
The present inventors made extensive study for preventing the fouling of a heat exchanger not only in a polyolefin production plant but also in a petroleum refining plant etc., and as a result they found that a specific nonionic surfactant is added to components mainly comprising hydrocarbons before passing through a heat exchanger thereby efficiently preventing the fouling of the heat exchanger, that is, suppressing a reduction in the performance of the heat exchanger, to achieve stable operation for a long time without sacrificing the speed of production, and the present invention was thereby arrived at.